FIG. 1 shows the anatomy of a normal human ear. A normal ear transmits sounds through the outer ear 101 to the tympani membrane (i.e., the eardrum) 102, which moves the bones of the middle ear 103, which in turn excites the cochlea 104. The cochlea (or inner ear) 104 includes an upper channel known as the scala vestibuli 105 and a lower channel known as the scala tympani 106, which are connected by the cochlear duct 107. In response to received sounds the stapes, a bone of the middle ear 103, transmits vibrations via the fenestra ovalis (oval window), to the perilymph of the cochlea 104. As a result, the hair cells of the organ of Corti are excited to initiate chemi-electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Some patients may have partially or completely impaired hearing for reasons including: long term exposure to environmental noise, congenital defects, damage due to disease or illness, use of certain medications such as aminoglycosides, or physical trauma. Hearing impairment may be of the conductive, sensorineural, or combination types.
One type of implant for patients with impaired hearing yet a fully functioning tympanic membrane and middle ear component(s) is a hearing implant that includes an implantable middle ear microphone. The middle ear microphone detects “sound” by sensing motion of middle ear component(s). The sensed motion of the middle ear may, for example, be processed by an implanted sound processor/cochlear stimulator into stimulus signals. The stimulus signals are adapted to stimulate nerves within the inner ear via a plurality of electrodes in an electrode array positioned in the inner ear (e.g., similar to an electrode array of a traditional cochlear implant).
Classical designs of a middle ear microphone have a resonance frequency that is outside the measured frequency range. In this manner, a flat transfer function across the measured frequency range may be obtained. An exemplary middle ear microphone attached to the umbo of the middle ear is described by Wen H. Ko, J. Guo, Xuesong Ye, R. Zhang, D. J. Young, MEMS Acoustic Sensor for Totally Implantable Hearing Aid Systems, IEEE Transactions on Biomedical Circuits and Systems, Volume 3, Issue 5, p. 277-285, 2008, which is hereby incorporated herein by reference in its entirety. The microphone disclosed by Ko et al. has a limited low frequency measurement range, being designed to work from 250 Hz to 8 KHz, and has a resonance frequency outside the used measurement range (200 Hz). The type of microphone disclosed by Ko et al. is an electrets microphone. Hence low frequencies below 200 Hz cannot be measured well. It also suffers from a large electrostatic force introduced by the electrets.
Another middle ear microphone design is disclosed in Darrin J. Young, Mark A. Zurcher, Wen H. Ko, Maroun Semaan, Cliff A. Megerian, Implantable MEMS Accelerometer Microphone for Cochlear Prosthesis, IEEE International Symposium on Circuits and Systems, ISCAS 2007, pages 3119-3122, 2007, which is hereby incorporated herein by reference in its entirety. The microphone of Young et al., is attached to the umbo of the middle ear, and is designed as an accelerometer (as opposed to a seismic sensor), with a targeted resonance frequency of 10 kHz. As the targeted frequency range is below 10 kHz, the microphone in Young et al. appears to be designed as a low pass filter, which is consistent with their description of the microphone as an accelerometer. Due to the high resonance frequency, the microphone of Young et al., is very limited in the low frequency range
Still another middle ear microphone is disclosed by Woo-Tae Park et al., Ultraminiature Encapsulated Accelerometers as a Fully Implantable Sensor for Implantable Hearing Aids, Biomed Microdevices, 9:939-9:949, 2007, which is hereby incorporated herein by reference in its entirety. The microphone disclosed by Woo-Tae Park et al. is a piezo-resistive microphone fixed at the stapes of the middle ear. Similar to the microphone of Young et al., the microphone is designed as an accelerometer (as opposed to a seismic sensor) with a resonance frequency in the range of 6 kHz. Due to the high resonance frequency, measurements are possible in the range of 900 Hz to 10 kHz, but are too limited in lower frequencies